Re: [PATCH v6 1/3] mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim

[Michal Hocko]

On Thu 13-04-23 10:40:32, Yosry Ahmed wrote:
> We keep track of different types of reclaimed pages through
> reclaim_state->reclaimed_slab, and we add them to the reported number
> of reclaimed pages.  For non-memcg reclaim, this makes sense. For memcg
> reclaim, we have no clue if those pages are charged to the memcg under
> reclaim.
> 
> Slab pages are shared by different memcgs, so a freed slab page may have
> only been partially charged to the memcg under reclaim.  The same goes for
> clean file pages from pruned inodes (on highmem systems) or xfs buffer
> pages, there is no simple way to currently link them to the memcg under
> reclaim.
> 
> Stop reporting those freed pages as reclaimed pages during memcg reclaim.
> This should make the return value of writing to memory.reclaim, and may
> help reduce unnecessary reclaim retries during memcg charging.  Writing to
> memory.reclaim on the root memcg is considered as cgroup_reclaim(), but
> for this case we want to include any freed pages, so use the
> global_reclaim() check instead of !cgroup_reclaim().
> 
> Generally, this should make the return value of
> try_to_free_mem_cgroup_pages() more accurate. In some limited cases (e.g.
> freed a slab page that was mostly charged to the memcg under reclaim),
> the return value of try_to_free_mem_cgroup_pages() can be underestimated,
> but this should be fine. The freed pages will be uncharged anyway, and we
> can charge the memcg the next time around as we usually do memcg reclaim
> in a retry loop.
> 
> Fixes: f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects
> instead of pages")
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
-- 
Michal Hocko
SUSE Labs